1. Field of the Invention
This invention relates to nondestructive testing of workpieces for flaws and more particularly to the testing for flaws in an elongated article while it is still hot from its fabrication.
2. Description of the Prior Art
In the manufacture of steel and other metal products it is desirable to inspect for flaws and imperfections. With current manufacture most inspection is done by or with the assistance of some type of detecting apparatus. Several popular testing apparatuses for detecting flaws utilize eddy currents.
In one type of eddy current apparatus, a high frequency power source induces eddy current flow in a workpiece being inspected. A detection assembly is positioned is proximity to the workpiece and they are relatively moved along a path of travel to locate any flaws that may be present in the workpiece.
Each detection assembly has one or more detecting coils which are inductively coupled with the workpiece and which effect the flaw detection. This detection results from the capability of a detection coil to produce output signals in response to variations in eddy current flow in the workpiece. These variations are caused by flaws in the workpiece. Accordingly, variations in output signals from the detection coils are indicative of flaws in the article being inspected.
These prior systems have been relatively successful in many applications but none have provided reliable inspection of hot, relatively large, pipe or bar stock immediately after manufacture. There have been proposals for testing immediately after fabrication but such proposed detection apparatus could not withstand fabrication temperatures in many applications.
Because of limitations in the hot inspection capabilities of prior inspection systems, all but very small steel pipe has, of necessity, been permitted to cool for approximately one hour before any flaw inspection was commenced. With present pipe-fabricating machinery, welded pipe can be produced at speeds of the order of 1600 feet of pipe per minute. If a pipe welder is malfunctioning, literally miles of defective pipe may be fabricated before flaw inspection reveals the existence of the malfunction in the welder.
Various proposals have been made for hot inspection of steel pipe, but known proposals have all had drawbacks. For example, proposals have been made to use heat-resistance materials, such as ceramics to protect detectors from heat, but these have not proved particularly practical and successful.
A principal shortcoming of prior propsals is they have not provided adequate heat transfer to maintain the detector transducers cool enough to assure reliable detection and adequate life spans. As a consequence, it has been difficult to obtain good signal-to-noise ratios which are important in obtaining high sensitivity in testing.
Others have proposed magnetic shield assemblies made primarily from metallic materials. Where metal materials have been proposed in such assemblies for positioning between a workpiece and a detector, longitudinal slotting has been considered essential. These slots are to be closed by electrically insulating materials. Such insulated slots were to prevent a loop-like flow of eddy currents which would mask workpiece eddy current flow and cause excessive losses in detection sensitivity. This slotting has been considered necessary because it has been believed that a circular conductive shield would destroy the operability of an inspection device.
If one attempts to apply one of these known slotted shields as a heat shield, one is confronted with a major problem in achieving effective cooling. Thermal expansion will inevitably result in coolant leakage between the metal and the electric insulator.
In a typical prior bar and tube inspector, both an exciter coil and a saturation coil surround the workpiece path of travel. The saturation coil magnetically saturates a workpiece to improve the sensitivity of the testing device. When a steel pipe is tested at a temperature above its Curie point, that is, about 1415.degree. F. for carbon steel, the saturation coils can be eliminated because the pipe is no longer magnetic.
While it has been recognized that the saturation coils may be eliminated, it has been considered necessary to provide fixed exciter coils which usually surround the workpiece path. Accordingly, in devices such as that shown in the DETECTOR CASE, or the earlier Sower et al U.S. Pat. No. 3,234,457, the amount of radial movement of a detector coil was limited by the surrounding exciter coil and its supporting structure.
There have been proposals to use exciter coils which do not surround a workpiece in inspection apparatus for inspecting only part of a pipe such as a weld. While there have been such proposals, exciter coils have been mounted in fixed positions relative to the paths of workpieces being inspected.
When a flaw inspection device surrounds a workpiece path, most repair and certainly any replacement can only be achieved when there is no workpiece along the workpiece path of travel. If the inspecting device is positioned to inspect pipe continuously rather than in sections, obviously the manufacturing operation must be interrupted to permit the desired repair or replacement.
Another disadvantage of prior inspection devices is they are relatively susceptible to a calamity that can result from what is known as a "cobble". From time to time, pipe coming from a continuous welder pipe forming machine may deviate rather appreciably from its designed and intended path of travel. It can deviate from a path of travel to the extent that it may strike a detecting assembly, even tearing it loose from its mounting. This is a cobble.
Characterization of the consequence of a cobble as a calamity does not seem an overstatement when one considers the possible and in fact probable consequences. The pipe can propel the dismounted detector along with it at speeds of the order of 30 miles per hour. Both the cables connecting the detector assembly to its associated electrical components and the components may be broken or damaged. In short, the entire flaw inspection apparatus can be destroyed by a cobble.
With the construction taught in the DETECTOR CASE and in those situations where only the weld is inspected with the construction taught in the Sower et al U.S. Pat. No. 3,234,457, the detector coils are protected to some extent from a cobble. They are protected because they are mounted in each case on biased arms which permit detector coil movement radially of the workpiece path of travel. The movement of the detector coils has been limited by the exciter coils which have surrounded both the workpiece and the detection coils. The exciter coils, as has been indicated, are fixed relative to the path of travel.
In small-size pipes where some hot inspection success has been achieved, there is another difficulty. This difficulty is in marking the hot metal at the location of a detected flaw. Conventional marking techniques are not readily adaptable, if adaptable at all, to use on hot products.